Well fluids including oil, gas, water and mud carry debris within the flow which gets deposited on the inner wall of a pipe or riser through which the fluid is flowing. The surface of the pipe or riser must be cleaned regularly in order to avoid these deposits from building up to a point where the drilling operation is affected.
A known well cleaning apparatus is described in U.S. Pat. No. 5,711,046 in which a tubular body is adapted for insertion into a well bore. The body has a plurality of cleaning pads mounted thereon. The cleaning pads have bristles on the outer surface thereof although scraper blades are also well known in the field. The cleaning pads are retained in position on the body by a collar which overlies a part of the cleaning pad.
As the body passes through a well tubular, the bristles or scraper blades contact the inner surface of the tubular and remove debris and deposits from the wall of the tubular thereby cleaning the inner surface.
Such a cleaning apparatus as described is useful in providing a cleaning operation where the inner diameter of the well tubular to be cleaned is constant. However, production tubing of a well is formed of a large number of tubulars connected together in a vertical string.
The end of each tubular is provided with an area of thickened diameter such as to produce a nipple on the annular tubular. When the tubulars are connected in the string, the inner bore of the resulting production tubing has interruptions which correspond in position to the nipples at the riser joints.
The cleaning apparatus as described above is unable to pass through the reduced diameter around the joints between a first riser and the subsequent riser in the string as the outer diameter of the body cannot be reduced by the appropriate amounts.